This invention relates to apparatus for driving the spindle of a machine tool with an induction motor, more particularly a novel torque control system for an induction motor utilized to drive the spindle of a machine tool or the like.
Where an induction motor is used to drive the spindle of a machine tool with variable speed, a change gear transmission, a pole number changing system of the induction motor, or a frequency converter which varies the frequency of AC current supplied to the motor has been used. With the change gear transmission and the pole number changing system, however, it is not only impossible to smoothly or steplessly control the motor speed but also requires a bulky and expensive speed control system. Accordingly, such system cannot enjoy the advantages of an AC motor, that is easiness of maintenance and low cost.
Furthermore, determination or orientation of the angular position of the spindle has been made with mechanical means instead of electrically controlling the motor shaft and such mechanical means complicates the construction of the spindle.